Metal-air batteries, particularly lithium-air, are considered a tantalizing goal in battery research. Use of atmospheric oxygen as one of the reactants in a primary or secondary battery would allow many benefits. These include much greater practical energy density, increased safety when fully charged, and reduced need for raw materials in manufacture.
However, to date, no technology has adequately addressed the major problems of metal-air batteries specifically, the air electrode. The reduction of atmospheric oxygen at ambient temperatures may result in partial reduction products, such as peroxides, superoxides, peroxyl radicals, and/or hydroxyl radicals, that may damage other components of the battery. The overpotential required to overcome the sluggish kinetics of the reaction may result in some of the battery's stored energy being released as heat, which is a practical as well as a safety consideration. Recharging such a cell may be even more problematic. In contrast to discharge, the overpotential required to affect the oxidation of, for example, peroxide on recharge may bring the electrode to higher, more damaging potentials.
Since recharging the battery requires the overpotential needed to affect the oxidation of metal-oxygen compounds (for example, lithium peroxide) higher than the equilibrium potential for the reaction, other problems may arise. Such high potentials (often greater than one volt versus a standard hydrogen electrode) may create a highly reactive, oxidative environment that can damage the air electrode, battery solvent, electrolyte and/or other materials that make up the battery. Further complications may include that waste heat may be generated from this overpotential, creating an environment which is even more likely to promote unwanted side reactions, damaging the battery and/or shortening its useful life. It is due to such phenomena that high energy density metal-air batteries (such as lithium-air and sodium-air batteries) have not yet realized their full promise.